Highlight all cells except those you want to be interactive, Format to mark as geschutzt und ausgeblendet. Then go to sheet Password protect and provide Passwords.  When you Password protect, only one box is ticked when you give your Password --- the 2nd from the top. (Nicht gesperrt Zellen auswahlen)

= Text(A2;"??/??")

=umwandeln(A2;von;auf)